1. Field of the Invention
The present invention relates to a boiling water type nuclear reactor use control rod which is for controlling an output of a nuclear reactor in a boiling water type nuclear reactor.
2. Convenitonal Art
Since reactor water in a boiling water type nuclear reactor contains such as oxygen and hydrogen peroxide which are produced through decomposition of water due to radiation exposure, it is commonly known that the reactor water is a negative potential as a corrosive environment against structural bodies in the reactor. One of conventional art for improving such corrosive environment is disclosed in JP-A-9-222495 (1997) in which hydrogen is injected into reactor water.
The method of injecting hydrogen is very effective for portions in a nuclear reactor in which water easily circulates, however, the effect of the method is thought to be a little for portions where water hardly circulates. For example, a clearance between a pin and a pin hole in a guide use roller for sliding a control rod in its axial direction is one of the portions where the water hardly circulates. If the current operation interval of the nuclear reactor is unchanged, a possible corrosion at such clearance causes no problem, however, the output of the nuclear reactor is increased and a stay interval of the control rod in the nuclear reactor is further prolonged, a possible corrosion at the clearance may cause a problem.
As one of methods of improving the environment at such clearance, it is conceived to exchange water at the clearance portion by simply enlarging the clearance. However, if the clearance between a pin and a pin hole is simply enlarged, the pin can not be set at a correct position, therefore, the sliding function of the control rod by the guide use roller can be extremely reduced.
An object of the present invention is to provide a boiling water type nuclear reactor control rod which can surely improve corrosive environment in a clearance portion at a guide use roller while maintaining a sliding function of the control rod by the guide use roller even when the stay interval of the control rod in the nuclear reactor is further prolonged.
In order to achieve the above object, in a boiling water type nuclear reactor use control rod according to the present invention which comprises a tie rod having substantially cross shaped cross section, a plurality of sheathes having substantially U shape cross section and attached to the respective sides of the tie rod, a plurality of neutron absorption rods disposed inside the respective sheathes, a handle which is disposed at one ends of the sheathes in the axial direction of the control rod and a lower portion supporting plate or a dropping speed limiter which is disposed at the other ends of the sheathes in the axial direction of the control rod, at least one of the handle, the lower portion supporting plate and the dropping speed limiter is provided with a sliding structure body (a guide use roller) which is constituted by a pin, a pin hole into which the pin is inserted and a roller which rotates around the pin in the axial direction, and a space (a groove) which causes water flow in a clearance between the pin and the pin hole is provided adjacent the clearance.
Preferably, the above space is provided at two positions one at the upstream side and the other at the downstream side in the axial direction. Further, preferably, the space is formed so as to extend near to the end portion of the pin.
According to the present invention, through the action of the space for causing water flow in the clearance, the water in the clearance portion can be effectively exchanged. Thereby, a possible increase of such as oxygen and hydrogen peroxide produced through water decomposition due to radiation exposure in the clearance can be prevented. Accordingly, even if a stay interval of the control rod in the nuclear reactor is prolonged, the corrosive environment at the clearance portion in the guide use roller can be surely improved.
Further, at a portion in the clearance where no space is formed, since the pin is positioned by the pin hole at a predetermined position, the sliding function of the control rod by the guide use roller can be maintained. Further, in case when cleaning is insufficient at the time of production of the control rod, through the water exchange in the clearance portion in the nuclear reactor a cleaning effect thereof can be expected.